Amine oxides are commonly used in cleaning compositions to boost and maintain suds formation. Such compositions include, for example, laundry, shampoo and dish washing detergent compositions.
Many methods for preparing amine oxide surfactants from the corresponding tertiary amine are known. Such methods involve the conversion of a tertiary amine in the presence of a strong oxidizing agent to the corresponding amine oxide. For example, tertiary amines can be reacted with hydrogen peroxide to yield a 30-40% aqueous solution of the corresponding amine oxide. Catalysts are commonly used to facilitate the reaction. Most of these methods, however, result in aqueous liquid formulations containing the amine oxide.
For shipping economy and for use in solid or granular detergent compositions, solid amine oxide surfactant formulations are more desirable than typical liquid amine oxide surfactant formulations. However, preparing such solid formulations has proven difficult over the years to the extent that most, if not all, of the commercially available amine oxide surfactant compositions are still in an aqueous liquid form. Such liquid formulations normally contain between 20-40% by weight of amine oxide and remain the primary source of amine oxide surfactants for use by manufacturers. It is not practical to evaporate or agglomerate these liquid formulations to obtain the solid amine oxides therein since such evaporation or agglomeration methods will result in pasty hydrates which are difficult to handle with conventional pumps and processing equipment. Spray drying the liquid formulations to obtain solid amine oxides is likewise undesirable in that the amine oxides steam distill thus forming unacceptable plume opacities during the spray drying operation.
There have been many attempts at preparing amine oxide surfactants in solid or granular formulations. One such method involves forming an aqueous solution of a salt of an amine oxide and an acid which is either an organic sulfonic acid or a fatty alcohol half-ester of sulfuric acid, and extracting the resulting salt from the solution with a water-immiscible organic solvent. Removal of the organic solvent from the resulting extract yields a dry and anhydrous amine oxide salt. Organic solvents for use in such a method can include methylene chloride, chloroform, methyl acetate, ethyl acetate, isopropyl acetate, methyl propionate, ethyl propionate, diethyl ether, dipropyl ether, ethyl propyl ether, ethyl butyl ether and methyl butyl ether. Another method for preparing amine oxide surfactants in a dry formulation involves preparing an amine oxide formulation wherein at least some of the amine oxide is in dihydrate form. In accordance with such a method, the amine oxide is formed by reacting a concentrated aqueous hydrogen peroxide with a tertiary amine in a reaction mixture which is maintained stirrable throughout the reaction by the use of an organic solvent which solubilizes the reaction mixture at the reaction temperature but permits precipitation of the resulting amine oxide at a lower temperature. After the reaction, solvent temperatures are reduced to induce amine oxide precipitation.
These methods for preparing amine oxide surfactants in solid formulations typically involve the use of organic solvents to induce precipitation of the amine oxide, or to otherwise control its solubility. However, the use of organic solvents in this manner has some serious disadvantages. First, the organic solvent must eventually be removed from the amine oxide material in an additional manufacturing step. Secondly, reducing the level of organic solvent residuals to an acceptable level in the amine oxide material can be difficult or economically unfeasible.
The foregoing considerations involving processes for preparing amine oxide surfactants in solid formulations indicate that there is a continuing need to provide improved processes for preparing such formulations. Accordingly, it is an object of the present invention to provide a novel process for preparing an amine oxide surfactant in a solid or granular formulation, and further to provide such a process that does not require the use of organic solvents or the additional process steps required to extract such organic solvents from the resulting solid amine oxide surfactant formulations.